The present invention relates to improvements in or relating to electrochromic systems.
As the energy performance of buildings and automobiles becomes an increasingly important design feature, strategies for optimising performance in this respect are receiving considerable attention.
An important aspect of the energy performance of the above relates to the incident radiation transmitted by the window area of a building. Such concerns are further complicated by the need to ensure occupant comfort. It is in this context that the electrochromic (EC) window technology has assumed increasing importance, the amount of incident radiation transmitted by such windows being electronically controllable. Effective implementation of EC window technology in buildings is expected to provide the following benefits:    1. Reduce adverse cooling effects.            Reduce cooling energy.        Down-size air conditioning plant.        Reduce peak electricity demand.            2. Increase beneficial effects of daylight.            Reduce lighting energy.        Reduce peak electricity demand.            3. Increase occupant comfort.            Increase thermal comfort.        Increase visual comfort.        
Even greater benefits would be expected to accrue in an automobile, where the ratio of glazed surface to enclosed volume is significantly larger than in a typical building. Specifically, effective implementation of EC window technology in automobiles is expected to provide the following benefits in addition to those in the built environment:    1. Increased motoring safety.            Reduced glare.        Mirror control.        Head-up display.        
EC technology is not limited to the applications described above. Others include privacy glass, angle-independent high-contrast large-area displays, glare-guards in electronic devices, electronic scratch pads.
Existing EC devices, including those commercially available, are non-optimal for the large glazing areas encountered in building and automotive applications and are based on technologies which are process and energy intensive. Therefore new EC technologies, resulting in improved device specification and which may be manufactured more easily at a lower cost, will be commercially important. It is noted in this context, that the current market for EC window technologies in buildings and automobiles is estimated world-wide at over $2 billion.
For an overview of these and related topics see the review Large-Area Chromogenics:Materials and Devices for Transmittance Control (Eds. Lampert and Granqvist), SPIE Institutes for Advanced Optical Technologies Series Vol. 4. Existing EC devices are found in one of the two categories outlined below. Firstly, there are those devices based on ion insertion reactions at metal oxide electrodes. To ensure the desired change in transmittance the required number of ions must be intercalated in the bulk electrode to compensate the accumulated charge. However, use of optically flat metal oxide layers requires bulk intercalation of ions as the surface area in contact with electrolyte is not significantly larger than the geometric area. As a consequence the switching times of such a device are typically of the order of tens of seconds.
Secondly, there are those devices based on a transparent conducting substrate coated with a polymer to which is bound a redox chromophore. On applying a sufficiently negative potential there is a transmittance change due to formation of the reduced form of the redox chromophore. To ensure the desired change in transmittance a sufficiently thick polymer layer is required, the latter implying the absence of an intimate contact between the transparent conducting substrate and a significant fraction of the redox chromophores in the polymer film.
As a consequence the switching times of such a device are, as above, typically of the order of tens of seconds.